Unless you buy aluminum tooling plate, it's very likely that .25 aluminum will not be very flat at all. And if it is, it will need to be well supported.
I work with corian a lot, and it's also not terribly flat. If you try to mill it flat, it'll curl up.
I'd recommend building a torsion box base, about 3" thick. My table is a 32"x60" torsion box, supported only at the ends, and even with 200 lbs on it (me), it'll stay flat to within 1/64".